AuthorTopic: AKG 414EB Picking Up Radio (Read 1039 times)

This is something I can not seem to find any information about so I thought I'd ask here. I will try to be brief.

A couple of years ago I purchased a used 414EB that was in reasonably poor condition. I sent it to AKG/Sennheiser for repair. Apparently there's only(at the time) one person left there that still has a collection of parts and the knowledge to service these. It's not a brass capsule but it's also not new probably an early 80's mic.

So they repaired it and it came back sounding like it should. I used it once and stored it in its AKG case. The next time I put it up for a session, there was a loud and pronounced radio station playing through this channel.

No, it's not in the wiring. No, it's not the cable, it is this mic and ONLY this mic. I have a lot of mics and NONE pick up radio in my room.

I would not be so fast discounting cabe and termination issues as you seem to be: 90% if not more of all RF interference into condenser mics emanate from faulty terminations or bad cable product.

You now may say: "but all my other mics work fine without RF when I use that cable!" To which I respond: a bad cable/termination will put a microphone at the borderline to be more susceptible to RF into audible RF, while a good cable/termination will prevent that.

Please open both connectors of the cable with which you experienced RF in your C414EB, and report how the ground, shield and connector housing are terminated. Also let us know the brand and connector configuration (2 or 3 conductors plus, shield). Then we figure out where to take it from here.

Does it catch hum if you put your hands close to it?In that case the screens or housing are not properly grounded.I even had issues with an early silver version with brass capsule, where the hum creeped in where the switches are located. The housing ground connection is on the bottom screws, maybe they need to be tightened or the tooth washers are missing. The screens got grounding wires soldered to them.The switch area was shielded with copper foil by me to get rid of the problem.

So this wire was installed by the previous owner, probably for the reason to remove hum. I replaced it, because it looked quite sloppy made.Without it no reliable grounding was present for the screens. Maybe it would have been enough to bend the screen a bit out to make contact with the housing, the clamping effect wasn't strong enough.But- the housing seems to be zink alloy, not that easy to make good electrical contact, even at the bottom screws, specially when the toothed washers are lost

Thanks for your replies. I can guarantee it is nothing to do with connections or cabling.

There is no hum. There is only a nice country/western station broadcast and of course the appropriate amount of "broadcast air".

One thing I did find when researching this some time back....The ground strap from the body to the mic-amp board was not tightened down and doing this LESSENED the effect of the broadcast. But I have been unable to trace any other paths to ground that could have been overlooked when it was repaired by AKG. My hope was to get a bit of info about where else to look in the mic itself. I know that Klaus doesn't want to work on these particular model of mics (at least that's what you told me So I'm left to my own devices.

On a side note I should give Klaus a call and get a quote on cleaning my U87 (again ). Thanks for the help gentlemen.

Before continuing to tap in the dark, can I ask again to identify mic cable terminations (see my earlier post)? You cannot make a reliable diagnosis of RF problems in mics without confirming proper ground and shield terminations and cable type.

Well, okay. I hope this is what you're asking....I have ,maybe 50 or 60 mic cables that are currently not in the repair bin. There are various types of cable....most are a couple of Belden numbers, I have some ProCo's, some Rapco's, a couple of oxygen free Horizon's, I have an UP OCC copper cable built by Charles @ Cathedral Pipes....ALL of them are terminated as standard XLR : 1=shield: 2=pos:3=neg......ALL have either Neutrix or Switchcraft connectors.

These all test correctly on my cable tester. They also ALL work on all 26 mics I currently use which include condensers both solid state and tube, various dynamics, and some ribbons.

The ONLY mic this phenomenon occurs with is the AKG 414EB. I do not live anywhere close to a broadcast tower of any sort. The electrical outlets in my recording area are firmly grounded and correctly phased ( I was an electrician for 35 years).

I hope that's the tell you needed. I can't think of anything else to describe.

Some of the cables I have built myself. I have a decent soldering station for this type of work. I don't use a 'wood-burner' or cheap solder. Some are custom built cables. Some are cables made by microphone manufacturers. I have a couple of actual Shure cables as well as a couple of Audio-Technica. I've opened them up and they are well made and trimmed out.

The question is, if the housing of the XLR plug (the 4th solder tap) is bridge connected to Pin1, shield, in your cables.On commercially available cables it's mostly not, according to my experience. This is done for good reason to avoid accidental ground loops with cables lying around. The downside of this practice not to connect the housing of the XLR is bad RF shielding under certain circumstances.This problem does not show up if the XLR is inserted into a microphone, because the XLR housing is connected to ground level by making contact to the microphone's housing.

You probably have a continuity tester, so check if all outer metal parts of the microphone and the inserted XLRf plug are tightly connected to ground (R<0.1Ohm).

For decoding AM radio a nonlinearity (harmonic distortion) in one of the microphone's components is necessary (besides that RF can creep in), probably the input FET. Maybe it's broken or not correctly biased.

Does the radio station go away when you put the microphone into a metal can?

Are the 2 blocking caps 22nF and the 2 little coils (Dr 1, Dr 2) at the output still present, not bridged?Some people think it's a good idea to deactivate or remove them for sound reasons, but it's not.

Here is an even simpler test to verify whether the cable is properly terminated: Hold the silver (not black!) head screen of the 414 firmly against the mic cable's XLR connector that is plugged into your mic pre: does the RF disappear?If so, the cable and/or its terminations are defective.

It's aways a good idea to hold service providers accountable.But getting cooperation from the service provider waiting 2 years to notify about a defect is unlikely: the longer you wait, the bigger the chance that the defect can no longer be directly linked to the service provider's incompetence.

By the way: It was most likely not Sennheiser who worked on that AKG mic.

Most definitely NOT serviced at Sennheiser or Neumann|USA! Despite assurances to the contrary, it still sounds like a shielding/grounding issue with the most likely suspects being the microphone cables missing their direct connection between pin 1 and the connector shell (4th terminal w.o. numerical designation), or poor shielding of the microphone housing parts. Some microphone types are far more susceptible to (RF-)interference than others, and in many cases additional measures for these are greatly ameliorating these problems: Installation of additional RF surpressing filters (chokes and capacitors), or using connectors with enhanced RF-screening. See also: http://www.neutrik.com/en/xlr/xlr-cable-connectors/emc-series/

No changes to touching the SILVER screen side to the cable connected at the preamp. No change when the mic goes in a can. (Folgers-large) The radio station is an FM station....strange. Beginning to suspect a broken FET. Although the mic does work and without the RF would be an excellent addition to the collection.

It was repaired at whatever was considered the AKG USA repair facility at the time. Somewhere around early 2015. If I can find the paper work I'll know for sure.

No changes to touching the SILVER screen side to the cable connected at the preamp.

Not the cable, but the connector at the end of the cable!

And, please forgive me, but I am getting frustrated to ask you the same question three times: what is the brand and type of cable? Is it a three conductor (best) or a two conductor with ground return over the shield (not so good)?

Sorry if I wasn't clear. Of course the connector ! My test cable is the UP OCC cable built by Charles @ Cathedral Pipes. I believe it is 2 conductor with a braided shield. Sorry if I didn't answer your question properly. Not my intention to cause frustration. The problem, however, does not lie within the cabling. I have several condensers that would exhibit at least a proportional amount of noise attributed to their sensitivity, and none have ANY noise with ANY cable I use.

Thanks for your help. I appreciate it and the effort. It is somewhere within the mic and its components. I'm scientific enough to test things completely and rule out things that are not cause for failure or problems. 35 years as a class A electrician taught me a lot about finding causes. I was hoping for an insight by asking my question here.

The post by Mr. Kai is the right course of action for me at this time. Thanks very very much.

Beginning to suspect a broken FET. Although the mic does work and without the RF would be an excellent addition to the collection.

Keep in mind a jfet transistor does not rectify a signal like the base of a bipolar transistor can. It's very unlikey the jfet is rectifing rf unless the leads or solder connections are suspect. Was it replaced? If so, solder flux or remaining dirt may be a cause.

Keep in mind a jfet transistor does not rectify a signal like the base of a bipolar transistor can. It's very unlikey the jfet is rectifing rf unless the leads or solder connections are suspect. Was it replaced? If so, solder flux or remaining dirt may be a cause.

Or maybe the ^Russians^?

It's the government spying on me through my Austrian equipment....

As to addressing this....I don't know what they replaced or repaired. I'm still looking for the paperwork. I talked with the tech who's name was 'Bob' (I think) and he was the one person who the repair department sent this mic to since he was the only one who had stockpiled parts for the EB and earlier 414's. I'm thinking he was going to retire around this time so reconnecting with his department might not be possible. It WAS AKG of some sort in the USA and if I could find the papers I would remember it better. This would have been in 2015.....I think.

Are you sure it's an FM station?It's almost impossible to decode FM into something other then noise, with the few electronic parts that live inside a mic, even if one tried on purpose.You need, basicly, a stable oszillator at about 100 MHz, tuned to the station, and more.Decoding AM is simple, can be done with almost any single active device that has nonlinearity.This is why the often very hot CB signals tended to creap into audio mixers.

The interferance not changing with the microphone shielded by a metal can or when touching its housing to the connector shell leads me to believe the cable to be acting as an antenna for the offending radio signal, which then enters either the microphone, or the input pre-amplifier stages. Use of the Neutric EMC-series connectors should ameliorate or eliminate this problem. Additional RF-filter components (ferrite beads, choke coils, capacitors) at the microphone's output and/or the pre-amplifier's input are an alternative possible solution.

Are you sure it's an FM station?It's almost impossible to decode FM into something other then noise, with the few electronic parts that live inside a mic, even if one tried on purpose.You need, basicly, a stable oszillator at about 100 MHz, tuned to the station, and more.Decoding AM is simple, can be done with almost any single active device that has nonlinearity.This is why the often very hot CB signals tended to creap into audio mixers.

I wasn't sure until a couple of days ago when I heard the call letters.

The interferance not changing with the microphone shielded by a metal can or when touching its housing to the connector shell leads me to believe the cable to be acting as an antenna for the offending radio signal, which then enters either the microphone, or the input pre-amplifier stages. Use of the Neutric EMC-series connectors should ameliorate or eliminate this problem. Additional RF-filter components (ferrite beads, choke coils, capacitors) at the microphone's output and/or the pre-amplifier's input are an alternative possible solution.

I still wouldn't bet that it's an FM station, but anyway:- Make sure there are proper ground connections everywhere.- Check all wire interconnects if nothing is loose.- Test the switches, does the radio station go away with certain setting?If yes, clean the switches.On the audio board:- Start by replacing all electrolytic and tantal caps, this should be done anyway.- Next candidate is the FET, then the 2 bipolar transistors.- While they're out you can check all the resistors for proper value.- Replace ceramic caps.- Check the film caps if you have a cap meter, specially those that are permanently in the audio path.

I still wouldn't bet that it's an FM station, but anyway:- Make sure there are proper ground connections everywhere.- Check all wire interconnects if nothing is loose.- Test the switches, does the radio station go away with certain setting?If yes, clean the switches.On the audio board:- Start by replacing all electrolytic and tantal caps, this should be done anyway.- Next candidate is the FET, then the 2 bipolar transistors.- While they're out you can check all the resistors for proper value.- Replace ceramic caps.- Check the film caps if you have a cap meter, specially those that are permanently in the audio path.

I thought it was weird to be picking up FM radio too, but the fact is, it is a station I regularly listen to....101.7 KINK in Portland where I live. I had not heard the call letters until a couple of days ago. The fact that the interference is strong enough to make out this makes the mic unusable.

I truly believe there is a ground lifted somewhere in this mic. When it came back from repair and started this nonsense I opened it up and the grounding strap was basically unscrewed. This cut the signal problem in half and now I'm looking for the other half.

I haven't found a loose connections other than that one at this time.....that doesn't mean there isn't one.

No difference in switch settings.

I'm taking it to a friends bench where there are the proper testing devices and we'll go from there.

One band-aid approach is to use a couple of those snap on ferrite bead cores in the plastic box and place them over the mic cable as it exits the XLR connector. That should remove the cable/contacts from the equation.